Fiber optic communication allows transmission of information from a source to a destination using optical fibers, which are flexible, transparent fibers made of thin glass silica or plastic that transmits light throughout the length of the fiber between the source and the destination. Fiber optic communications allows for the transmission of data over longer distances and at higher bandwidth than other known forms of communications.
Optical networks generally use multiplexing to exploit the large bandwidth of optics. Multiplexing enables several virtual channels to be formed on a single fiber. Therefore, multiplexing several optic signals increases the connectivity of a network. There are two methods of multiplexing signals, time division multiplexing and wavelength division multiplexing. Time division multiplexing (TDM) multiplexes several signals by establishing different virtual channels using different time slots. Wavelength division multiplexing (WDM) multiplexes the signals by having different virtual channels use different wavelengths.
Bi-directional passive optical network (PON) systems may use two different spectrums during optical communication. One spectrum is used for sending information and the other is used for receiving information, resulting in bi-directional communication through one fiber. Long haul and metro WDM systems generally use a pair of fibers for transmission in two directions, so there is no need to use two different bands and a simple loopback is possible. WDM systems may include a multiplexer and a demultiplexer. The multiplexer combines the signals at a transmission side, and the demultiplexer separates the signals at a receiver side. Some systems include a multiplexer that combines signals and separates the signals.
Optical loopback is difficult for a wavelength division multiplexing (WDM) optical communication system using two separate spectrums/bands for optical communication. Optical loopback is generally accomplished by looping back the transmit signal to the receive port using a fiber patch cord and/or an attenuator. For a WDM system having a transmit spectrum A different from a receive spectrum 1B, simply using a fiber patch cord and/or an attenuator to loopback an optical signal is generally not possible, since a receiving filter typically blocks all the transmit wavelengths. Instead, the WDM system typically converts the optical transmit signals to electrical signals and then converts the electrical signals back to optical signals having the correct receiving wavelengths. This solution, however, requires a large number of optical transceivers with different wavelengths. Alternatively, a WDM transceiver module with reversed wavelength assignment can be installed into the same system to perform a loopback test. That solution is relatively expensive as well and adds uncertainty from the test module themselves.